Virus-induced diseases in agronomically important crops have cost farmer s a great loss of income due to reduced yields. Traditionally, virus diseases have been controlled by breeding for host plant resistance or by controlling insects that transmit diseases. Chemical means of protection are not generally possible for most viruses, and where possible are not generally practical. It has been known for many years that viral symptoms can be reduced in virus-infected plants by prior inoculation with a mild strain of the same virus, a phenomena known as cross-protection, as described by Sequeira, L., Trends in Biotechnology, 2, 25 (1984). Cross-protection is considered successful if the disease symptoms of the superinfecting (the more virulent) virus can be delayed or suppressed. There are several disadvantages to applying this type of cross-protection to the field situation:
1) application of the mild strain virus to entire fields is usually not practical,
2) the mild strain might undergo mutation to a more highly virulent strain,
3) the protecting strain might interact synergistically with a non-related virus causing a severe pathogenic infection,
4) a protecting virus in one crop may be a severe pathogen in another crop, and
5) a protective strain may cause a significant loss of yield in itself.
One proposed solution to these disadvantages has been to introduce a single viral gene into the host plant genome to cross-protect, rather than infect with an intact virus. This single gene cross-protection strategy has already been proven successful using the coat protein gene from Tobacco Mosaic Virus (TMV-CP). AS reported by Abel, P. P., et.al., Science, 232, 738 (1986 ), transgenic tobacco plants, expressing TMV mRNA and coat protein (CP), demonstrated delayed or suppressed symptom development upon infection with TMV. TMV-CP transgenic tomato plants have been described by Nelson, R. S., et.al., Bio/Technology, 6, 403 (1988), to show evidence of protection from TMV as well as three strains of Tomato Mosaic Virus (ToMV).
Numerous viruses exist for which resistance is desired. Maize Dwarf Mosaic Virus causes a somewhat variable mosaic or yellow streaking and occasional stunting in maize. Early infections can result in severe symptoms including premature death. The virus is transmitted mechanically and in nature is spread by several plant-feeding insect species, including corn leaf aphid, greenbug, and green peach aphid. Strain A of MDMV can overwinter in Johnsongrass, and as a result has become a recurrent problem in areas where Johnsongrass is a common weed. Combined infections with Maize Chlorotic Mottle Virus can cause severe chlorosis and stunting, and can produce corn lethal necrosis, a severe syndrome observed in certain areas of Nebraska. Thus, there is a continuing need for genes, plant transformation vectors, and transformed plant materials providing resistances to pathogenic viruses such as MDMV. Unfortunately, while certain plant viruses, such as Tobacco Mosaic Virus, have coat protein genes that are found on subgenomic RNA and are therefore relatively easy to identify and clone for use in engineered cross-protection, potyviruses have a different genome organization which makes it more difficult to identify the coat protein gene.